1. Field of the Invention
The present invention relates to a cooling apparatus for a gas turbine moving blade and also to a gas turbine equipped with the cooling apparatus and applied to an electric power plant or the like and, more particularly, to a closed-loop cooling type cooling apparatus for a moving blade which achieves a higher cooling efficiency by supplying in parallel a cooling medium from the inside of a turbine rotor to respective moving blades constituting a plurality of stages and recovering the cooling medium, which has been used for cooling, so as to achieve higher energy efficiency.
2. Discussion of the Background
In recent years, it has become especially important to improve operating efficiency of a gas turbine used at an electric power plant or the like from the standpoint of economy, namely, a reduced amount of fuel supplied for combustion and of environmental preservation, namely, reduced emission of CO.sub.2 and NOx.
Hitherto, a combined cycle power generating system composed of a hot gas turbine and a steam turbine has been considered as a power generating system with the highest efficiency. In the combined cycle power generating system, increasing the temperature at the inlet of the gas turbine is directly related to higher thermal efficiency of power generation. For this reason, technological development efforts have been made to fulfill goals including increasing the combustion gas temperature at the inlet of the gas turbine to 1500.degree. C. or higher from the current temperature of 1300.degree. C. which has already exceeded the melting point of metallic materials.
In such a high-temperature gas turbine, the sections exposed to high-temperature gas are generally cooled by circulating high-pressure air supplied from an air compressor. For cooling the moving blades, in particular, which are fixed to a turbine rotor and placed in the field of a strong centrifugal force, so-called "open-loop cooling" has been used wherein cooling air is introduced from a cooling air passage, which is formed at the center of the turbine rotor, into a plurality of stages of moving blades so as to convectively cool the inside of the moving blades, then the air which has been used for the cooling is let out into a mainstream combustion gas.
FIG. 13 illustrates an example of a conventional gas turbine cooling unit which employs the open-loop cooling technology described above. A turbine rotor 15 of the example shown in the drawing is constituted by connecting a plurality of discs 6, 7 and 8 which have, for example, first through third stages of implanted moving blades 3, 4 and 5 between a front disc 1a made integral with a front shaft 1 and a rear disc 2 separate from the front disc 1a. They are connected together with spacers 12 and 13 disposed in correspondence with the predetermined positions of stationary blades 9, 10 and 11 by using a plurality of tie-bolts 14 which are parallel to the central axis portion of the turbine rotor 15. On the side of the outer peripheries of the tie-bolts 14 in the turbine rotor 15, there are spaces 16, 17, 18, 19, 20 and 21 respectively defined between the front disc 1a and the disc 6 of the first-stage moving blade, between the respective discs 6, 7 and 8 and the spacers 12 and 13, and between the rear disc 2 and the third-stage disc 8. These spaces 16, 17, 18, 19, 20 and 21 are in communication with spaces 28, 29, 30 and 31 on the side of the inner peripheries thereof through grooves 22, 23, 24, 25, 26 and 27 at the connecting portion formed by the tie-bolts 14.
When the gas turbine, such as shown in FIG. 13, is operated, a part of combustion air supplied from an air compressor is used as a cooling medium, and the cooling air (arrow a) serving as the cooling medium is led from the interior of the front shaft 1 into the spaces 28, 29, 30 and 31 in sequence on the inner periphery side. The cooling air flows outward in the radial direction in the spaces 16, 17, 18, 19, 20 and 21 on the outer periphery side through the grooves 22, 23, 24, 25, 26 and 27 and then flows into an internal cooling passage (such as a meandering channel or the like which is not shown) of the moving blades, or into the gap between the disc 8 of the last stage (i.e. the third stage) and the spacer 13 clamping it and the rear disc 2. After the cooling air flows in the internal passage or the like to carry out convective cooling, it is jetted into a mainstream combustion gas (arrow b).
In the case of such an open-loop cooling type gas turbine, however, the low-temperature air a used for cooling is jetted into the high-temperature mainstream gas b to to be mixed therewith. This leads to a drop in the temperature of the mainstream gas b, an increase in the loss of the flow attributable to the mixing, and a loss in the pumping power relative to the cooling air a in the rotation field, etc., resulting in a drop in the turbine output due to cooling. The drop in the turbine output leads to lower power generating efficiency. Further, even if an air compressor of the same size is used, the increase in the cooling air a causes a decrease in the combustion air with a consequent drop in the power of the gas turbine.
If the temperature of the gas turbine is further increased in the future with the above-mentioned problems unsolved, it is likely that more air for cooling the blades will be necessary, and the cooling will markedly restrict improvement of the efficiency achieved by raising the temperature, or the combustion air to be used for a low-NOx combustor will be insufficient, preventing the increase of the gas temperature.
As a solution to such problems, there has been proposed an improvement in the air-cooled gas turbine, or a "closed-loop cooling type steam-cooled gas turbine" in which water vapor or the like is used as the cooling medium and recovered after being used for the cooling. For instance, Japanese Patent Laid-open Publication No. HEI 8-14064 discloses an art wherein air or vapor is employed as the cooling medium and the cooling medium is recovered after it is used for cooling, thereby preventing the thermal efficiency from decreasing. Japanese Patent Laid-open Publication No. HEI 7-301127 discloses an art wherein steam is used as the cooling medium in most cases, and the cooling medium after it has been used for cooling is recovered so as to improve the efficiency of the gas turbine.
In the closed-loop cooling type gas turbine cooling apparatus in the prior art described above, however, has a series cooling structure in which a plurality of cooling elements such as a plurality of stages are cooled in sequence. In this type of serial cooling structure, there is a likelihood that a high cooling effect is obtained only at a portion in contact with air on the upstream side, and the cooling effect deteriorates toward the downstream side. For example, there has been known a case where the trailing edges of blades, which are small portions of the blades, are insufficiently and unevenly cooled, resulting in cooling difficulties.
To solve the problem of the cooling difficulties, a cooling structure is conceivable wherein a plurality of stages are provided with a cooling medium arranged in parallel. In this case, however, successful layout of the members for controlling the flow of the cooling medium need to be achieved. For example, the turbine rotor runs at high speed, and the members provided in the turbine rotor to control the flow are subjected to an extremely strong centrifugal force. Therefore, the members controlling the flow must have adequately high structural strength. Specifically, discs or the like are used as in the conventional structure. However, high load would be applied to the circumferential portions of the discs. In addition, sliding motion or the like between the high-speed rotating section and a stationary section would be necessary, and therefore, special attention must be paid to the design of the sealing of the cooling medium.
Hitherto, there has been known no construction designed to perform parallel cooling by adopting the closed-loop cooling system and a gas turbine equipped with such cooling system, taking the foregoing respects into account. Furthermore, there has been no desirable art related especially to a multi-stage parallel cooling structure or to a combination of a steam cooling system and an air cooling system and a gas turbine equipped with such cooling system.